1. Field of the Invention
The present invention relates generally to a model car, and more specifically to a differential for a model car, which can eliminate power loss of the model car.
2. Description of the Related Art
In a model car without a differential, the inner driving wheel and the outer driving wheel are forced to rotate at the same speed such that the model car is difficult to corner. Thus, a differential is generally used in a power transmission system of a model car to allow the rotational speed of the outer driving wheel to be larger than that of the inner driving wheel.
FIG. 1 shows a conventional differential 1 for a model car, which comprises a housing 2 having a chamber 3 therein, four notches 6 recessed in a periphery of the chamber 3 and arranged in a crisscross manner, two large bevel gears 4a and 4b received in the chamber 3 and respectively connected with output shafts 8a and 8b, a spider 5 having four ends respectively inserted in the notches 6, and four small bevel gears 7 respectively sleeved on one end of the spider 5 and engaged with the large bevel gears 4a and 4b. As a result, when the power generated by an engine is transmitted to a main gear 9, a part of the power will be transmitted to the output shaft 8a connected with the main gear 9, and other part of the power will be transmitted to the output shaft 8b through the large bevel gear 4a, the small bevel gears 7, and the large bevel gear 4b such that the driving wheels of the model car can be allowed to rotate at different speeds by the engagement transmission of the large bevel gears 4a and 4b and the small gears 7.
However, when the model car corners, the inner driving wheel may suspend in midair due to the centrifugal force and spin such that the reduction gear ratio between the inner driving wheel and the outer driving wheel will exceed the initial set value. In order to prevent the power loss of the model car caused by the spinning inner driving wheel, the spider 5 will be moved to make the small bevel gears 7 and the large bevel gears 4a and 4b engage with each other; nevertheless, the ends of the spider 5 are respectively inserted in the notches 6 of the housing 2 such that they will rub the periphery walls of the notches 6 when the spider 5 is moved. After the differential 1 is used for a long time, the notches 6 will be gradually widened because of the constant friction, causing the looseness of the spider 5. As a result, the small bevel gears 7 can't be tightly engaged with the large bevel gears 4a and 4b when the model car corners, resulting in idle spinning of the inner driving wheel and the power loss of the model car.